Integrated circuits are typically manufactured on semiconductor wafers that are then sawed (diced) into individual die. Typical microelectronic devices experience internal heating during operation that may reach a level such that the device no longer functions properly. To avoid such overheating, the die package may be thermally coupled to heat dissipation hardware (e.g., a heat sink and/or heat spreader). Attaching a heat sink to the die package requires that two solid surfaces be brought into intimate contact. The solid surfaces are not smooth enough to allow the desired contact. This is due to the microscopic hills and valleys of the solid surfaces as well as to macroscopic non-planarity in the form of a concave, convex, or twisted shape. As two such solid surfaces are brought together, only a small percentage of the surfaces make physical contact, with the remainder separated by a layer of interstitial air. Some heat is conducted from the die through the points of physical contact, but the majority must be transferred through the interstitial air layer. Since air is a relatively poor thermal conductor, the interstitial air layer is replaced with a TIM to increase the joint thermal conductivity and thus improve heat flow across the interface. The TIM brings the die package into good thermal contact with the heat dissipation hardware.
Various types of thermally conductive materials may be used as the TIM to eliminate air gaps from the interface including greases, reactive compounds, elastomers, and pressure sensitive adhesive films. TIMs are designed to conform to surface irregularities, thereby eliminating air voids, thus improving heat flow through the interface.
FIGS. 1A and 1B illustrate an exploded view of TIMs used in a typical microelectronic device in accordance with the prior art. As shown in FIG. 1A, a die package 105 mounted on a printed circuit board (PCB) 110 coupled by TIM 115 to a heat dissipation device, shown as heat sink 120. The heat sink 120 is typically aluminum and has fins as shown. The heat sink 120 may also include a fan, not shown (active heat sink). TIM 115 brings the die package 105 into intimate contact with the heat sink 120.
As shown in FIG. 1B, the microelectronic device may include a heat spreader 116 to improve the efficiency of heat transfer from the die package 105 to the heat sink 120. For such a device, a first TIM 115 may be used to improve thermal contact between the die package 105 and the heat spreader 116. A second TIM 117 may be used to improve contact between the heat spreader 116 and the heat sink 120.
FIG. 2 illustrates an arrangement of a non-fusible particle filler material within the polymer matrix of a TIM in accordance with the prior art. The polymer matrix may be a material that can be applied as a paste such as a dispensable syringe or by screen-printing. The polymer matrix may also act as an adhesive to bond the two mating parts together. The non-fusible particles, such as most metals, benefit from a high thermal conductivity, however a thermal flow path through the TIM is limited by the point-to-point contact of the particles as shown by the arrows. Non-fusible particles refer to particles that will not melt and flow during packaging assembly process, reliability testing, and product operation and so remain as point contacts with each other. This provides thermal conductivity through the TIM that is limited to point-to-point percolation, resulting in a thermal bottleneck through the non-fusible particles.
The phenomenon of percolation describes the effects of interconnections present in a random system, here the number of filler particles that are randomly in point contact with each other to allow thermal conduction. Normally, to improve conduction limited by percolation, the amount of filler could be increased until a threshold amount is reached and heat conduction, due to the filler, transitions to a sufficiently high value. The degree of filler required to reach this transition level may be too high and can overpower the properties desired from the polymer binder such as low contact resistance. Another problem is that for some metal particles in contact with some polymer binders, the bare particle filler can poison the polymer cure such as by hindering or blocking the curing agent.
To address these concerns, a PSH TIM has been developed that includes fusible particles as well as filler particles in a silicone polymer matrix material. The fusible particles melt during the assembly process and can therefore wet the filler particles or self-coalesce. Thereby the average particle size grows creating long continuous heat transfer pathways that alleviate the thermal bottleneck of percolation. The fusible particles may be materials such as solder-like materials that melt below approximately 300° C. The filler particles may be non-fusible materials with melting points well above 300° C., such as aluminum at 660° C., silver at 961° C., copper at 1084° C., gold at 1064° C., etc.
Silicone exhibits certain characteristics (e.g., low glass transition temperature and low moisture absorbency) that make it suitable as a binder matrix for PSH TIMs. For purposes of this disclosure, low glass transition temperature is approximately 25° C. and low moisture absorbency is approximately 1% or less by weight. Other materials may exhibit such characteristics and therefore may likewise be suitable as a binder matrix for PSH TIMs. Such materials may provide better adhesion and lower contact resistance than silicones and similar materials.